Silicon nitride is of great interest for structural and electronic applications because of its excellent high temperature strength, good thermal shock resistance, good wear resistance and chemical inertness. Silicon nitride materials have been traditionally fabricated by one of three methods: (1) the direct nitridation of silicon powder at high temperatures, (2) densification of commercially available silicon nitride powder at high temperatures by the addition of metal oxide powders known as sintering aids that promote densification of the silicon nitride by providing a liquid phase during sintering and (3) densification of commercially available silicon nitride powder at high temperatures and pressures, a process known as hot pressing. These silicon nitride materials have been used in structural applications such as cutting tools and gas turbines.
Although silicon nitride ceramics having satisfactory mechanical properties at ambient and modestly high temperatures are available, many are not suitable for high temperature applications because their toughness, wear resistance and strength are often degraded at temperatures greater than 1000.degree. C. In the case of cutting tool inserts, wear resistance is a particularly important property. Undesirable phase transformations within the insert and chemical reactivity with the metal being worked on cause a deterioration in properties as the temperature rises at the high feeding and rotating speeds used for milling and turning operations.
Various methods have been proposed for improving the high temperature properties of silicon nitride ceramics. For example, U.S. Pat. Nos. 4,093,687 and 4,124,402 disclose a process for preparing high density polycrystalline silicon nitride suitable for high temperature structural applications by hot pressing silicon nitride and magnesium silicide in the absence of oxide additives. Varying amounts of alpha- and .beta.-phase silicon nitride are present in the product, depending on the temperature used. U.S. Pat. No. 4,407,971 discloses a sintered silicon nitride body having excellent mechanical strength at high temperatures and made from a mixture of silicon nitride, aluminum oxide, yttrium oxide, aluminum nitride and small amounts of metal silicide. Japanese unexamined patent application JP 58-176109 discloses alpha-type silicon nitride having a large specific surface area that is useful for heat resistant structural materials. The silicon nitride is produced by combusting amorphous silicon nitride and/or silicon diimide in the presence of a high melting point metal silicide such as Mo silicide. JP 58-064269 discloses the preparation of a sintered silicon nitride body having excellent strength and hardness at high temperatures from a mixture of 1-20 wt. % Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3, 5-40 wt. % of a metal silicide and the balance silicon nitride. However, none of these references discloses the production of wear resistant ceramic articles with a high alpha-content silicon nitride phase and also containing silicides of Fe, Co or Ni that are primarily composed of high metal content silicides.